Patterns of nitrogen export from a seasonal freezing agricultural watershed during the thawing period.

The objectives of this study were to investigate water, ammonium nitrogen (NH4+-N), and nitrate nitrogen (NO3--N) export processes during the thawing period in a watershed with heavy agricultural activities and to evaluate contributions of N (i.e., NO3--N and NH4+-N) from different source areas under different climate conditions. Experiments were conducted within the 75km2 agricultural Heidingzi watershed in northeast China. The thawing period was divided into four stages: early-melt, late-melt, rain-on-melt, and post-melt. Drainage regions (DRs) were separated into three types. The processes of water and N discharge from soil into rivers were monitored in these DRs during the thawing periods of 2014, 2015, and 2016. Results show that the processes of water and N discharge were not synchronous during the thawing period. Variations in discharge concentrations of NH4+-N and NO3--N during the thawing period were mainly affected by the flushing effect, which was controlled by the physical state of the surface water (snow or ice) and the melt rate of frozen soil. Contributions of N export from the DRs varied under different land uses and climate conditions during the thawing period. NO3--N export was mainly from maize fields. Thawing stages with high NO3--N export were always accompanied by higher discharge rates. NH4+-N export mainly occurred during the early-melt and late-melt stages and from riverside rural regions.